2010-04-13 16:07:12
Troubleshoot BGP
BGP is a path-vector routing protocol. Routes are tracked in terms of the AS they pass through,
and routers avoid loops by rejecting routes that have already passed through their AS.

Definitions
Synchronization — Before iBGP can propagate a route, the route must be learned from an IGP.
Split Horizon — BGP will not advertise a route out the interface from which it was learned.
Path Selection — Routers avoid loops by rejecting routes that already include their AS. BGP attributes are used to determine the best route to a destination.

BGP Attributes

Order

Preference

Description

0. Synchronized

TRUE

Use only routes that meet the synchronization requirement

1. Weight_______________

Highest

Administrative override, a router will use the interface with the highest weight when multiple paths exist. Not propagated to other routers.

2. Local Preference

Highest

Used internally to pick path out of AS and is propagated to its internal peers.

3. Self Originated

TRUE

Prefer paths that originated on this router.

4. AS-Path

Shortest

Minimize AS-hops, prefer the route that has the shortest AS Path.

5. Origin

i<?

Prefer stability, where IGP is lower than EGP, and EGP is lower than incomplete.

6. Multi-Exit Discriminator (MED)

Lowest

Advertised to external neighbors in an attempt to influence path selection in your AS.

Only the “best” routes as defined by the attributes make it into the routing table.

sh ip route bgp

BGP States
BGP cycles through five states as it runs:
Idle – Searching for neighbors
Connect – TCP three-way handshake complete with neighbor
Open Sent – BGP Open message has been sent
Open Confirm – Response received
Established – BGP neighborship is established
Remember: Established is good, anything else is bad.

Troubleshoot BGP States
– If a neighbor does not progress from idle, look for a next-hop address that cannot be reached.
– If the neighbor stays active, the neighbor is not responding as expected, so look for things that affect this reply. For instance,the peering IP address or AS number may be incorrect, the neighbor may be misconfigured, or authentication may be misconfigured.
BSCI p.406, TSHOOT p.229